feat: add support for webhook retries

Add a new field `.spec.webhooks[].retries` to specify the number of
retries when calling a webhook.

Signed-off-by: Joseph Kwasniewski <kwasniewski@gmail.com>

docs/gitbook/usage/webhooks.md

@@ -41,6 +41,7 @@ Spec:
       - name: "start gate"
         type: confirm-rollout
         url: http://flagger-loadtester.test/gate/approve
+        retries: 5
       - name: "helm test"
         type: pre-rollout
         url: http://flagger-helmtester.flagger/
@@ -72,6 +73,7 @@ Spec:
       - name: "send to Slack"
         type: event
         url: http://event-recevier.notifications/slack
+        retries: 3
         metadata:
           environment: "test"
           cluster: "flagger-test"
@@ -122,6 +124,8 @@ Event payload (HTTP POST):
 The event receiver can create alerts based on the received phase 
 (possible values: `Initialized`, `Waiting`, `Progressing`, `Promoting`, `Finalising`, `Succeeded` or `Failed`).
 
+The webhook request can be retried by specifying a positive integer in the `retries` field.
+
 ## Load Testing
 
 For workloads that are not receiving constant traffic Flagger can be configured with a webhook,